1. Field of the invention
The present invention relates to modem technology, for instance for VDSL (Very high bit-rate Digital Subscriber Line) applications.
2. Description of the Related Art
In modem operation, a so-called bit swap algorithm is generally used during ShowTime (i.e., session of data payload transmission) to improve the quality of modem performance.
A bit swap process is used in discrete multi-tone (DMT) or OFDM (orthogonal frequency division multiplexing) based modulations (including wireless solutions) with a feedback channel to maximize the QoS (quality of service) when other methods are failing. In wireline modem applications using bit swap processing is mandatory both in ADSL and VDSL solutions.
The Bit Swap algorithm operates at the receiver side since there it can monitor the SNR (signal-to-noise ratio) and has all the information needed to make a bit swap decision. The target of bit swap processing is to minimize the BER (bit error rate) (or, equivalently, to maximize the noise margin). This is done by moving bits between different tones or by changing tone gains. Usually, the number of operations (moving bits between different tones or change the tone gain) is very limited to reduce the overhead channel rate. The literature on bit swap processing is not particularly extensive.
Bit swap processing must comply with a list of constraints. These constraints are usually related to a power spectral density (PSD) mask or to aggregate total power figures. Specific constraints exist in some standard to mitigate cross talk to other modems or simply related to the bit swap protocol.
Providing a good bit swap algorithm involves solving several problems. Some of the most significant of those problems are highlighted below:
the bit swap algorithm should be efficient, since e.g., 2 k (two thousand) tones may need to be processed in VDSL. A higher speed of the algorithm means that more bit swap requests can be finalized in the same period of time (higher reaction speed);
the bit swap algorithm should give selective priority to different operations to be performed, because the most urgent operations should be performed first. The criteria for assigning priorities to operations are based on BER (or Noise Margin) considerations. This is because the multi-tone margin (usually defined to be equal to or close to the minimum of the tone noise margins) should be increased in the fastest way even if the temporary bit and gain distribution is not optimum and then caused to converge towards the optimum one with lower priority (this means higher priority to fast and “dirty” approach, lower priority to slow and “clean” approach);
the bit swap algorithm should be stable. If a noise level variation occurs, it should be able to optimize bit and gain per tone distribution and it should be able to converge towards a certain bit and gain distribution (this means that after a certain number of bit swap requests no other bit swap request should occur);
the bit swap algorithm should give optimal or quasi-optimal results in terms of final BER once the algorithm is converged to a stable solution (i.e., multi-tone margin);
the bit swap algorithm should not be subject to blocking: sometimes, due to power constrains (or PSD constraints), it is not possible to perform any operation even if a better solution exists. This condition should be avoided;
the Bit Swap algorithm should be compatible with bit swap protocol constraints like, for example, the limited number of operations or discrete steps in the variation of the current gain, indicated gi. It also should be compatible with power constraints (if existing).